In a mobile (cellular) communications network, mobile devices (also known as User Equipment (UE) or mobile terminals, such as mobile telephones) communicate with remote servers or with other mobile devices via base stations. In their communication with each other, mobile devices and base stations use licensed radio frequencies, which are typically divided into frequency bands and/or time blocks. Depending on various criteria (such as the amount of data to be transmitted, radio technologies supported by the mobile device, expected quality of service, subscription settings, etc.), each base station is responsible for controlling the transmission timings, frequencies, transmission powers, modulations, etc. employed by the mobile devices attached to the base station. The scheduling decision can be modified every transmission time interval, e.g. as frequently as 1 ms. In order to minimise disruption to the service and to maximise utilisation of the available bandwidth, the base stations continuously adjust their own transmission power and also that of the mobile devices. Base stations also assign frequency bands and/or time slots to mobile devices, and also select and enforce the appropriate transmission technology to be used between the base stations and the attached mobile devices. By doing so, base stations also reduce or eliminate any harmful interference caused by mobile devices to each other or to the base stations.
In order to be able to communicate via the base stations, mobile devices need to monitor the control channels operated by the base stations. One of these control channels, the so-called Physical Downlink Control Channel (PDCCH) carries the scheduling assignments and other control information. The PDCCH serves a variety of purposes. Primarily, it is used to convey the scheduling decisions to individual mobile devices, i.e. scheduling assignments for uplink and downlink communication.
The information carried on the PDCCH is referred to as downlink control information (DCI). The format of the DCI can vary depending on the purpose of the control message.
An additional Physical Control Format Indicator Channel (PCFICH) is transmitted by the base station to indicate the size of the PDCCH (e.g. the number of orthogonal frequency-division multiplexing (OFDM) symbols occupied by the PDCCH). Using OFDMA, the mobile devices are allocated blocks comprising a specific number of subcarriers for a predetermined amount of time. These are referred to as physical resource blocks (PRBs) in the LTE specifications. PRBs thus have both a time and a frequency dimension. One PRB consists of 12 consecutive subcarriers for one slot (0.5 ms) in duration. The PRB is the smallest element of resource allocation assigned by the base station. LTE, radio frames are divided into 10 subframes, each subframe being 1.0 ms long. Each subframe is further divided into two slots, each of 0.5 ms in duration. Slots consist of either 6 or 7 ODFM symbols, depending on whether the normal or extended cyclic prefix is employed.
Physical control channels, such as the PDCCH, are transmitted on an aggregation of one or several consecutive Control Channel Elements (CCEs), where a control channel element corresponds to nine Resource Element Groups (REGs). Each REG has four Resource Elements (REs).
When a mobile device is first switched on or when it arrives in an area served by a base station, it will look for the location of the control channels in the frequency band(s) used by that base station. For example, the mobile device needs to check all possible combinations of locations and formats of the PDCCH, and the DCI formats and act on those messages. Since the decoding of all possible combinations would require the mobile device to make many PDCCH decoding attempts, 3GPP defined an alternative approach for LTE, according to which, for each mobile device served by the base station, a limited set of CCE locations are set where a PDCCH may be placed. The set of CCE locations in which the mobile device may find its PDCCH can be considered as a ‘search space’, for example, as described in section 9.1.1 of the 3GPP TS 36.213 standard.
In LTE the search space is a different size for each PDCCH format. Moreover, separate dedicated and common search spaces are defined, where a dedicated search space is configured for each UE individually, while all mobile devices are informed of the extent of the common search space.
It has been decided, as part of the 3GPP standardisation process, that downlink operation for system bandwidths beyond 20 MHz will be based on the aggregation of a plurality of component carriers at different frequencies. Such carrier aggregation (CA) can be used to support operation in a system both with and without a contiguous spectrum (for example, a non-contiguous system may comprise component carriers at 800 MHz, 2 GHz, and 3.5 GHz). Whilst a legacy mobile device may only be able to communicate using a single, backward compatible, component carrier, a more advanced multi-carrier capable terminal would be able to simultaneously use the multiple component carriers.
Carrier aggregation can be particularly beneficial in a heterogeneous network (HetNet), even when the system bandwidth is contiguous, and does not exceed 20 MHz because multiple carriers enable interference management between different power class cells as well as open access and closed subscriber group (CSG) cells. Long-term resource partitioning can be carried out by exclusively dedicating carriers to a certain power class of cell (Macro/Pico/CSG).
3GPP have considered introducing a so-called Enhanced Physical Downlink Control Channel (ePDCCH) which supports Carrier Aggregation (CA), including new carrier types (also known as CA Enhancement), Coordinated Multiple Point Transmission/Reception (CoMP) and Downlink Multiple In Multiple Out (DL MIMO) technologies. Advantageously, the ePDCCH supports:                increased control channel capacity;        frequency-domain Inter-Cell Interference Control (ICIC);        improved spatial reuse of control channel resource;        beamforming and/or diversity;        new carrier type(s);        Multicast-Broadcast Single Frequency Network (MBSFN) subframes;        legacy User Equipment (UE) coexisting on the same carrier;        frequency-selective scheduling; and        mitigation of inter-cell interference.        